System and method for noise cancellation in a signal processing circuit

ABSTRACT

A system and method for noise cancellation in a signal-processing circuit (e.g., an analog-to-digital converter circuit). Various aspects of the present invention may comprise inputting a first input signal and a digital input signal to the signal-processing circuit. The digital input signal may, for example, comprise a digital dither signal or other processor control signal. The signal-processing circuit may, for example, output a signal comprising a first signal component that is primarily a function of the first input signal and a second signal component that is primarily a function of the digital input signal. The second signal component may be estimated based on estimated behavior of the signal-processing circuit in response to the digital input signal. The estimated second signal component may, for example, be substantially removed from the signal-processing circuit output signal.

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BACKGROUND OF THE INVENTION

Signal-processing circuits, such as analog-to-digital convertercircuits, may under particular circumstances, exhibit tonal behavior.That is, the signal-processing circuits may generate tones, which may beundesirable for an intended application. For example, a delta-sigmamodulator circuit may, in certain circumstances, output quantizationnoise, including idle-channel noise. Also for example, a delta-sigmamodulator circuit may, in certain circumstances, exhibit periodic (e.g.,tonal) behavior. Even in circumstances where such noise is not directlyperceivable by a user, such noise may adversely affect circuitperformance.

Various attempts have been made to reduce tonal behavior insignal-processing circuits. For example, in various analog-to-digitalconverter circuits, attempts have been made to introduce a random signalinto the circuit to disrupt periodic circuit behavior. An example ofthis may be found in “Delta-Sigma Data Converters, Theory Design andSimulation,” by Norsworthy et al., and published by IEEE Press, whichshows introducing a random analog signal into a delta-sigma modulatorcircuit.

Introducing such a random analog signal into a circuit may not be anoptimal solution, depending, for example, on the particular circuit andpractical circuit design considerations. Accordingly, attempts have beenmade to introduce a random digital signal into a circuit to disruptperiodic circuit behavior. An example of this may be found in U.S. Pat.No. 6,577,257 to Brooks, the content of which is hereby incorporatedherein by reference in its entirety. The U.S. Pat. No. 6,577,257 shows asystem and method for introducing a digital dither signal into adelta-sigma modulator circuit to disrupt periodic behavior.

Introducing a digital signal (e.g., a digital dither signal) into asignal-processing circuit to disrupt periodic behavior or to controlother noise-producing aspects of a signal-processing circuit may resultin decreased circuit performance and undesirable components in an outputsignal of the signal-processing circuit. That is, introducing such adigital signal into a signal-processing circuit may solve one problemwhile creating others.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

Various aspects of the present invention provide a system and method fornoise cancellation in a signal-processing circuit. Such asignal-processing circuit may, for example, comprise ananalog-to-digital converter circuit or other signal-processing circuit.

Various aspects of the present invention may comprise inputting a firstinput signal and a digital input signal to the signal-processingcircuit. The digital input signal may, for example, comprise asignal-processing control signal. Such a signal-processing controlsignal may, for example and without limitation, comprise a digitaldither signal or other digital signal that controls an aspect of thesignal-processing circuit behavior.

The signal-processing circuit may, for example, generate amulti-component signal comprising a first signal component and a secondsignal component. The first signal component may, for example, beprimarily due to the signal-processing circuit processing the firstinput signal. The second signal component may, for example, be primarilydue to the signal-processing circuit processing the digital inputsignal. The generated signal may also comprise other signal components.

The second signal component may, for example, be estimated based onestimated or known behavior of the signal-processing circuit in responseto the digital input signal. For example, the signal-processing circuitmay generally process the digital input signal according to a known orestimated transfer function. The second signal component may, forexample, be estimated by applying the digital input signal to the knownor estimated transfer function.

The estimated second signal component may be subtracted, or otherwiseremoved, from the multi-component signal generated by thesignal-processing circuit to yield an output signal from which thesecond signal component has been substantially removed.

These and other advantages, aspects and novel features of the presentinvention, as well as details of illustrative aspects thereof, will bemore fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a method for noise cancellation ina signal-processing system, in accordance with various aspects of thepresent invention.

FIG. 2 is a diagram showing an analog-to-digital converter circuitimplementing noise cancellation, in accordance with various aspects ofthe present invention.

FIG. 3 is a diagram showing a signal-processing system implementingnoise cancellation, in accordance with various aspects of the presentinvention.

FIG. 4 is a diagram showing a first order analog-to-digital convertercircuit implementing noise cancellation, in accordance with variousaspects of the present invention.

FIG. 5 is a diagram showing a second order analog-to-digital convertercircuit implementing noise cancellation, in accordance with variousaspects of the present invention.

FIG. 6 is a plot illustrating an exemplary system performance increasefrom implementing noise cancellation in an analog-to-digital convertercircuit, in accordance with various aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a flow diagram illustrating a method 100 for noisecancellation in a signal-processing system, in accordance with variousaspects of the present invention. The signal-processing system may, forexample, comprise any of a large variety of signal-processing systems.For example, the signal-processing system may comprise various audio,video and data processing systems. For example, the signal-processingsystem may comprise an analog-to-digital circuit (e.g., a delta-sigmamodulator circuit).

Various aspects of the method 100 may be described below using anexemplary signal-processing scenario comprising an analog-to-digitalconverter. Such an exemplary signal-processing scenario orsignal-processing system configuration are offered herein forillustrative purposes only (i.e., for providing a specific example ofbroader aspects). Accordingly, the scope of various aspects of thepresent invention should by no means be limited by characteristics ofany exemplary system or scenario described herein, including thoseinvolving analog-to-digital converter circuitry.

The method 100 begins at step 110. The method 100 may be initiated byany of a large number of triggering events. For example, the method 100may execute whenever power and/or a signal is applied to a circuitimplementing the method 100. Also, for example, the method 100 mayexecute automatically in response to a particular detected noisesituation. Additionally, for example, the method 100 may execute inresponse to an operator command. Accordingly, the scope of variousaspects of the present invention should not be limited bycharacteristics of particular initiating conditions or circumstances.

The method, at step 120, may comprise inputting a first signal to thesignal-processing system. The first signal may, for example, be analogor digital. In an exemplary signal-processing system comprising ananalog-to-digital converter circuit, the first signal may, for example,comprise an analog signal that is to be converted to a digital signal bythe analog-to-digital converter circuit. For example and withoutlimitation, the signal-processing system may comprise a delta-sigmamodulator configuration of an analog-to-digital circuit. In general,step 120 may comprise inputting a first signal to the signal-processingsystem. Accordingly, the scope of various aspects of the presentinvention should not be limited by characteristics of a particular firstsignal or signal-processing system architecture.

The method, at step 130, may comprise inputting a second signal to thesignal-processing system. The second signal may, for example, comprise adigital signal. Such a digital signal may, for example, be input to thesignal-processing system to control a behavior of the signal-processingsystem while the signal-processing system is processing the firstsignal, which was input to the signal-processing system at step 120.

In an exemplary signal-processing system comprising an analog-to-digitalconverter circuit, the second signal may, for example, comprise adigital dither signal (or other processing control signal), which may beinput to the analog-to-digital converter circuit to disrupt tonalbehavior of the analog-to-digital converter circuit. Such tonal behaviormay, for example, result from a delta-sigma modulator processing a lowfrequency or low amplitude analog input signal.

In general, step 130 may comprise inputting a second signal to thesignal-processing system. Accordingly, the scope of various aspects ofthe present invention should not be limited by characteristics of aparticular second signal or signal-processing system architecture.

The method 100, at step 140, may comprise outputting an output signalfrom the signal-processing system. The output signal may comprise, forexample, a first component that is primarily due to thesignal-processing system processing the first input signal, and a secondcomponent that is primarily due to the signal-processing systemprocessing the second input signal. It should be noted that in certainscenarios, various components of the output signal may not be cleanlyseparable (e.g., due to signal-processing non-linearities in thesignal-processing system). In such instances, respective portions of theoutput signal may still be identifiable as being primarily due to thefirst input signal and/or the second input signal.

For example and without limitation, consider a scenario where thesignal-processing system comprises an analog-to-digital convertercircuit, the first input signal comprises an analog signal to beconverted to a digital signal, and the second input signal comprises adigital dither signal (or other processing control signal). In theexemplary scenario, the analog-to-digital converter circuit may outputan output signal that comprises a first component that is primarily dueto the analog-to-digital converter circuit converting the analog inputsignal to a digital signal, and a second component that is primarily dueto the analog-to-digital converter circuit processing the digital dithersignal.

In general, step 140 may comprise outputting an output signal from thesignal-processing system based on the first and second input signals.Accordingly, the scope of various aspects of the present inventionshould not be limited by characteristics of a particular output signalor components thereof.

The method 100, at step 150, may comprise estimating a portion (orcomponent) of the output signal from the signal-processing system thatis primarily due to the signal-processing system processing the secondsignal input to the signal-processing system. That is, a portion of theoutput signal from the signal-processing system may be primarily due tothe signal-processing system processing the second input signal, whichwas input to the signal-processing system at step 130. Step 150 maycomprise estimating such a portion of the output signal.

For example and without limitation, consider an exemplary scenario wherethe signal-processing system comprises an analog-to-digital convertercircuit and the second input signal comprises a digital dither signal(or other processing control signal). The analog-to-digital convertercircuit may, for example, output a digital output signal, a portion ofwhich is primarily due to the analog-to-digital converter circuitprocessing the digital dither signal. In this example, step 150 maycomprise estimating the portion of the signal output from theanalog-to-digital converter circuit that is due to the analog-to-digitalconverter circuit processing the digital dither signal.

Estimating a portion of the output signal from the signal-processingsystem that is primarily due to the signal-processing system processingthe second signal input to the signal-processing system may be performedin a variety of ways. For example, a transfer function that exactly orapproximately characterizes the functional relationship between thesecond signal input to the signal-processing system and the signaloutput from the signal-processing system may be developed. Such atransfer function may, for example, be developed analytically orexperimentally. Such a transfer function may also, for example, beadapted in real-time based on monitored behavior of thesignal-processing system. Such a transfer function may, for example, belinear or non-linear, or analog or digital. Such a transfer functionmay, for example, comprise a digital approximation of analog systembehavior. Accordingly, the scope of various aspects of the presentinvention should not be limited by characteristics of a particulartransfer function of characteristics of determining such a transferfunction.

Once a transfer function has been developed or determined, the portionof the output signal from the signal-processing system that is primarilydue to the signal-processing system processing the second signal inputto the signal-processing system may be estimated (or known) by applyingthe transfer function to the second signal input to thesignal-processing system. In other words, once the output response ofthe signal-processing system to the second signal input is known, onemay apply such a response to the second input signal to determine aportion of the output from the signal-processing system resulting fromthe second input signal.

In an exemplary scenario where the signal-processing system comprises ananalog-to-digital converter circuit (e.g., a delta-sigma modulatorcircuit), and the second signal input to the signal-processing systemcomprises a digital dither signal (or other processing control signal),the transfer function may characterize the effect that a given digitaldither signal will have on the output of the analog-to-digitalconverter. Accordingly, to estimate (or know) a component of the outputsignal from the analog-to-digital converter that is due to theanalog-to-digital converter processing a known digital dither signal,the transfer function may be applied to the known digital dither signal.

In general, step 150 may comprise estimating a portion (or component) ofthe output signal from the signal-processing system that is primarilydue to the signal-processing system processing the second signal inputto the signal-processing system. Accordingly, the scope of variousaspects of the present invention should not be limited bycharacteristics of a particular signal-processing system, particularsecond input signal, or particular way of estimating the portion of theoutput signal from the signal-processing system that is primarily due tothe signal-processing system processing the second input signal.

The method 100, at step 160, may comprise removing the portion (orcomponent) of the output signal estimated in step 150 from the outputsignal of the signal-processing circuit. That is, step 150 may haveestimated the portion of the output signal from the signal-processingcircuit that was primarily due to the second input signal. Step 160comprises removing such an estimated portion of the output signal fromthe output signal.

Consider the exemplary signal-processing circuit configuration, wherethe signal-processing system comprises an analog-to-digital convertercircuit, the first input signal comprises an analog signal that theanalog-to-digital converter circuit is to convert to a digital signal,and the second input signal comprises a digital dither signal (e.g.,input to the analog-to-digital converter circuit to disrupt tonalbehavior). Step 150 may have, for example, estimated the portion of theoutput signal from the analog-to-digital converter circuit that was dueto the analog-to-digital converter circuit processing the digital dithersignal. Step 160 may then, for example, remove the estimated portionfrom the output signal, thereby substantially removing the effect of thedigital dither signal on the signal output from the analog-to-digitalconverter circuit.

Step 160 may, for example, remove the estimated portion of the outputsignal from the output signal in a variety of ways, including analog ordigital processing. For example, step 160 may utilize digital hardwaresubtracting circuitry to perform sample value modification. Also, forexample, step 160 may utilize a processor executing softwareinstructions. Additionally, for example, step 160 may utilize analogcircuitry to effect such signal component removal.

In general, step 160 may comprise removing the portion (or component) ofthe output signal estimated in step 150 from the output signal of thesignal-processing circuit. Accordingly, the scope of various aspects ofthe present invention should not be limited by characteristics of aparticular method or apparatus for removing a signal component from acomposite signal.

The method 100, at step 170, continues performing further functionality.For example, step 170 may lead to continued processing of the first andsecond inputs in accordance with steps 120–160. For example, step 170may initiate performing steps to analyze the current behavior of thesignal-processing circuit, and make adjustments to the steps 120–160 asneeded. Accordingly, the scope of various aspects of the presentinvention should not be limited by characteristics of signal processingthat occurs after performing various steps of the method 100.

FIG. 2 is a diagram showing an analog-to-digital converter circuit 200implementing noise cancellation, in accordance with various aspects ofthe present invention. As mentioned previously, the analog-to-digitalconverter is utilized herein to provide particular illustrative examplesof broader aspects of the present invention. Accordingly, the scope ofvarious aspects of the present invention should not be limited bycharacteristics of an analog-to-digital converter.

The exemplary analog-to-digital converter circuit 200 comprises an A/Dconverter module 210 and a filter module 250. The A/D converter module210 comprises an analog input 212, a digital dither input 214, and adigital output 216. The filter module 250 comprises a digital input 252,a digital dither input 254, and a digital output 256. The digital input252 of the filter module 250 is communicatively coupled to the digitaloutput 216 of the A/D converter module 210.

The A/D converter module 210 may, for example, receive an analog inputsignal at the analog input 212. The analog input signal may, forexample, comprise an analog signal that the A/D converter module 210 isto convert to a digital signal. The A/D converter module 210 may, forexample, comprise a delta-sigma modulator circuit for converting theanalog input signal to a digital signal.

The A/D converter module 210 may, for example, receive a digital dithersignal (or other processing control signal) at the digital dither input214 of the A/D converter module 210. The digital dither signal may, forexample, be introduced to disrupt tonal behavior of the A/D convertermodule 210. For example, as mentioned previously, the A/D convertermodule 210 may comprise a sigma-delta modulator circuit, which mayexhibit tonal behavior under particular operating circumstances.Alternatively, for example, a digital signal received at the digitaldither input 214 may control other processing aspects of the A/Dconverter module 210.

The A/D converter module 210 may, for example, output a digital outputsignal at the digital output 216 of the A/D converter module 210. Such adigital output signal may, for example, comprise a first signalcomponent (or portion) based primarily on the analog input signalreceived at the analog input 212. The functional relationship between asignal input at the analog input 212 and output from the digital output216 may, for example, be characterized by a signal transfer function.The digital output signal may also, for example, comprise a secondsignal component (or portion) based primarily on the digital dithersignal input at the digital dither input 214. The functionalrelationship between a signal input at the digital dither input 214 andoutput from the digital output 216 may, for example, be characterized bya digital dither transfer function.

As mentioned previously, the digital input 252 of the filter module 250may be communicatively coupled to the digital output 216 of the A/Dconverter module 210. Accordingly, the filter module 250 may, forexample, receive the digital output signal from the digital output 216of the A/D converter module 210. The received signal may, for example,comprise the first and second signal components discussed above.

The filter module 250 may, for example, receive a digital dither signal(or other processing control signal) at the digital dither input 254 ofthe filter module 250. The digital dither signal received at the digitaldither input 254 may, for example, be based on the digital dither signalreceived at the digital dither input 214 of the A/D converter module210. To this end, the digital dither input 254 of the filter module 250may, for example, be communicatively coupled to the digital dither input214 of the A/D converter module 210. The respective digital ditherinputs 254, 214 may, for example, be conductively coupled.

The filter module 250 may, for example, output a digital signal at thedigital output 256 of the filter module 250. Such a digital outputsignal may, for example, be based on respective signals received at thedigital input 252 and digital dither input 254 of the filter module 250.For example, as mentioned previously, the filter module 250 may receive,at the digital input 252, the digital output signal from the digitaloutput 216 of the A/D converter module 210. Also, as mentionedpreviously, the digital output signal from the digital output 216 of theA/D converter module 210 may comprise a first signal component, basedprimarily on the analog input to the A/D converter module 210, and asecond signal component, based primarily on the digital dither input tothe A/D converter module 210.

The filter module 250 may, for example, process the digital outputsignal from the A/D converter module 210 to substantially remove thesecond signal component of the digital output signal. The filter module250 may then output a digital output signal reflective of the digitaloutput signal received from the A/D converter module 210 with the secondsignal component substantially removed.

For example, the filter module 250 may determine or estimate the secondsignal component of the digital output signal. The filter module 250may, for example, process the digital dither input with a transferfunction indicative of the digital dither transfer function of the A/Dconverter module 210. Such a transfer function may, for example, be anexact duplicate of the digital dither transfer function of the A/Dconverter module 210. Alternatively, for example, such a transferfunction may be substantially or approximately equivalent to the digitaldither transfer function of the A/D converter module 210, or even insome instances, a rough approximation of the digital dither transferfunction of the A/D converter module 210.

The filter module 250 may, for example, implement such determination orestimation with a first sub-module. The first sub-module may, forexample, comprise digital hardware, software, analog hardware or hybridcircuitry to implement such a determination or estimation. Accordingly,the scope of various aspects of the present invention should not belimited by characteristics of particular methods or circuitry that maybe utilized to determine or estimate a signal component.

Once the filter module 250 determines or estimates the second signalcomponent of the digital input signal received from the A/D convertermodule 210, the filter module 250 may remove the second signal componentfrom the digital input signal. The filter module 250 may, for example,utilize a second sub-module to perform such removal.

The second sub-module may perform such component removal using a varietyof techniques and circuitry. For example and without limitation, thesecond sub-module may use digital signal subtraction circuitry tosubtract the estimated or known second signal component from the digitalinput signal. The second sub-module may, for example, utilize digital,analog or hybrid circuitry to perform such component removal.Accordingly, the scope of various aspects of the present inventionshould not be limited by characteristics of a particular method orcircuitry for removing a signal component from a signal.

It should be noted that the filter module 250 may be coupled with, orcomprise components of, an additional filter. For example and withoutlimitation, in an exemplary A/D converter scenario, the filter module250 may be coupled with, or comprise components of, a decimation filter.Accordingly, the scope of various aspects of the present inventionshould not be limited by potential additional features or functionalityof the exemplary filter module 250.

FIG. 3 is a diagram showing a signal-processing system 300 implementingnoise cancellation, in accordance with various aspects of the presentinvention. The exemplary system 300 may comprise a signal-processingmodule 310 and a filter module 350. The exemplary signal-processingmodule 310 may comprise a first input 312, a digital input 314 and anoutput 316. The exemplary filter module 350 may comprise a first input352, a digital input 354 and an output 356. The first input 352 of thefilter module 350 may, for example, be communicatively coupled to theoutput 316 of the signal-processing module 310.

The exemplary signal-processing module 310 may, for example, receive aninput signal at the first input 312. Such an input signal may be analogor digital. The signal-processing module 310 may process the inputsignal. Such processing may, for example, be characterized by a signaltransfer function 320.

The signal-processing module 310 may comprise any of a variety ofsignal-processing entities. For example and without limitation, thesignal-processing module 310 may comprise an analog-to-digital converter(e.g., an A/D converter based on a delta-sigma modulator circuitconfiguration). In such an exemplary scenario, the signal-processingmodule 310 may receive an analog input signal at the first input 312,process the analog input signal according to the signal transferfunction 320 (comprising aspects of an analog-to-digital convertertransfer function), and output the processed signal at the output 316.The signal-processing module 310 may, for example, comprise a variety ofanalog and digital processing modules. Accordingly, the scope of variousaspects of the present invention should not be limited bycharacteristics of particular circuit configurations orsignal-processing techniques.

The signal-processing module 310 may output the processed signal at theoutput 316. A signal output by the signal-processing module 310 at theoutput 316 may comprise a combination of a plurality of signals. Forexample and without limitation, the summing node 335 shown in thediagram of the signal-processing module 310 illustrates that the signaloutput from the signal-processing module 310 at the output 316 maycomprise a signal component corresponding to the input signal processedaccording to the signal transfer function 320 in addition to othersignal components.

The signal-processing module 310 may, for example, receive a digitalinput signal (e.g., a digital dither signal or other processing controlsignal) at the digital input 314. The signal-processing module 310 may,for example, utilize such a digital input signal to control processingaspects of the signal-processing module 310. For example and withoutlimitation, the digital input signal may comprise a digital dithersignal that may, when processed by the signal-processing module 310,disrupt tonal behavior of the signal-processing module 310.

The exemplary system 300 illustrated in FIG. 3 comprises a digitalsignal generator module 340. An output of the digital signal generatormodule 340 may, for example, be communicatively coupled to the digitalinput 314 of the signal-processing module 310. The digital signalgenerator module 340 may, for example, generate a digital processorcontrol signal that, when processed by the signal-processing module 310,controls signal-processing behavior of the signal-processing module 310.As an example, the generated digital processor control signal (e.g. adigital dither signal), when processed by the signal-processing module310, may result in reduced noise (e.g., tonal noise) in thesignal-processing module 310. In general, the digital signal generatormodule 340 may generate any of a large variety of digital processorcontrol signals. Accordingly, the scope of various aspects of thepresent invention should not be limited by characteristics of aparticular digital processor control signal.

The signal-processing module 310 may, for example, process the digitalinput signal received at the digital input 314. Such processing may, forexample, be characterized by a digital transfer function 330. Thesignal-processing module 310 may, for example, process the digital inputsignal according to the digital transfer function 330, and output aresulting signal at the output 316. As discussed previously, a signaloutput at the output 316 of the signal-processing module 310 maycomprise a plurality of signal components. As an example, an outputsignal may comprise a first signal component resulting from thesignal-processing module 310 processing a signal received at the firstinput 312 according to the signal transfer function 320, and a secondsignal component resulting from the signal-processing module 310processing a signal received at the digital input 314 according to thedigital transfer function 330.

Additionally, the summing node 325, shown in the exemplarysignal-processing module 310, illustrates that the signal-processingsystem 300 may have internal sources of noise. Such noise may, ofcourse, be added to signals at various points in the signal-processingsystem 300. Accordingly the signal output by the signal-processingmodule 310 at the output 316 may also comprise a noise component due toany of a large variety of noise sources within and without thesignal-processing module 310.

As mentioned previously, the output 316 of the signal-processing module310 may be communicatively coupled to the first input 352 of the filtermodule 350. That is, the filter module 350 may receive the output signalfrom the signal-processing module 310, which may comprise a plurality ofsignal components. One of such components may be due to thesignal-processing module 310 processing the digital input signalaccording to the digital transfer function 330.

The filter module 350 may also receive a digital input signal at thedigital input 354 of the filter module 350. Such a digital input signalmay, for example, be based on the digital input signal input provided tothe signal-processing module 310. The digital input signal received atthe digital input 354 of the filter module 350 may comprise, forexample, a signal based on a signal-processing control signal (e.g., adigital dither signal) provided to the digital input 314 of thesignal-processing module 310.

The digital input 354 of the filter module 350 may, for example, becommunicatively coupled to the digital signal generator module 340discussed previously. The digital input 354 of the filter module 350may, for example, be communicatively and/or conductively coupled to thedigital input 314 of the signal-processing module 310.

The filter module 350 may, for example, comprise a first sub-module 360having an input and an output. The input of the first sub-module 360may, for example, be communicatively coupled to the digital input 354 ofthe filter module 350. Accordingly, in an exemplary scenario where thedigital input 354 of the filter module 350 is communicatively coupled tothe digital input 314 of the signal-processing module 310, the input ofthe first sub-module 360 may receive the same digital input signal thatis applied to the digital input 314 of the signal-processing module 310.

The first sub-module 360 may, for example, process the digital inputsignal according to a digital transfer function. Such a digital transferfunction may, for example, be based on the digital transfer function 330of the signal-processing module 310. For example, the digital transferfunction of the first sub-module 360 may be equal to or approximatelyequal to the digital transfer function 330 of the signal-processingmodule 310. Also, for example, the digital transfer function of thefirst sub-module 360 may be a rough approximation of the digitaltransfer function 330 of the signal-processing module 310. In general,the digital transfer function of the first sub-module 360 may be basedon the digital transfer function 330 of the signal-processing module310. Accordingly, the scope of various aspects of the present inventionshould not be limited by an exact relationship between the digitaltransfer function of the first sub-module 360 and the digital transferfunction 330 of the signal-processing module 310.

In an exemplary scenario, the first sub-module 360 may receive the samedigital input signal that is applied to the digital input 314 of thesignal-processing module 310. In such an exemplary scenario, the firstsub-module 360 may, for example, process the digital input signalaccording to a digital transfer function that is based on the digitaltransfer function of the signal-processing module 310. In such ascenario, the first sub-module 360 may generate a signal indicative ofthe signal component of the signal-processing module 310 output 316 thatis due to the signal-processing module 310 processing the digital inputsignal.

The filter module 350 may, for example, comprise a second sub-module 370comprising a first input, a second input and an output. The first inputmay, for example, be communicatively coupled to the first input of thefilter module 350. The second input may, for example, be communicativelycoupled to the output of the first sub-module 360.

The second sub-module 370 may, for example, output a signal that isbased on a difference between a signal received at the first input and asignal received at the second input. The second sub-module 370 mayperform such differencing functionality in a variety of ways, including,but not limited to, utilizing digital subtracting circuitry.

For example, the second sub-module 370 may receive, at the first input,the output signal from the signal-processing module 310, where suchoutput signal may comprise a plurality of signal components. The secondsub-module 370 may also receive, at the second input, the output signalfrom the first sub-module 360, which, as discussed above, may beindicative of a signal component of the output signal from thesignal-processing module 310. Accordingly, the second sub-module 370 mayprocess the respective input signals and output a signal indicative ofthe output signal from the signal-processing module 310 with the signalcomponent removed.

The output of the second sub-module 370 may, for example, becommunicatively coupled to the output 356 of the filter module 310.

FIG. 4 is a diagram showing a first order analog-to-digital convertercircuit 400 implementing noise cancellation, in accordance with variousaspects of the present invention. For example and without limitation,the first portion 410 of the exemplary circuit 400 may correspond to aspecific example of one of many possible circuit configurations for thesignal-processing module 310 illustrated in FIG. 3. Also, for exampleand without limitation, the second portion 450 of the exemplary circuit400 may correspond to a specific example of one of many possible circuitconfigurations for the filter module 350 illustrated in FIG. 3.Accordingly, the scope of various aspects of the present inventionshould by no means be limited by aspects of the specific circuit 400illustrated in FIG. 4.

The exemplary A/D converter circuit 400 receives an analog input signalV_(in), at the positive input of a difference circuit 412. Thedifference circuit 412 receives, at its negative input, a feedbacksignal. An analog signal indicative of the difference between the analoginput signal V_(in) and the feedback signal is applied to a firstfunctional block 415 having a transfer function G(z). The output of thefirst functional block 415 is input to a Quantizer 420, which convertsthe analog signal to discrete (or digital) form. The output of theQuantizer 420 is input to a summing circuit 430, which also receives adigital dither signal. The summing circuit 430 adds the output of theQuantizer 420 to the digital dither signal and outputs the signalD_(out). The signal D_(out) is applied to a second functional block 435having a transfer function H(z). The output of the second functionalblock 435 represents the feedback signal discussed previously, which wasinput to the negative input of the difference circuit 412.

The transfer function characterizing the functional relationship betweenV_(in), and D_(out) is generally G(z)/(1+G(z)H(z)) and may be referredto as the signal transfer function (STF). The transfer functioncharacterizing the functional relationship between the digital dithersignal and D_(out) is generally 1/(1+G(z)H(z)) and may be referred toherein as the noise transfer function (NTF). The output signal from thefirst portion 410 (D_(out)) includes a first signal component resultingfrom processing V_(in) by the STF and a second component resulting fromprocessing the digital dither signal by the NTF. Additionally, D_(out)may comprise various other signal components (e.g., a signal componentdue to Quantizer noise).

The digital dither signal is input to a third functional block 460having a transfer function ˜NTF(z). As mentioned previously the noisetransfer function NTF characterizes the functional relationship betweenthe digital dither input and D_(out). In the exemplary circuit 400, thethird functional block 460 transfer function ˜NTF(z) is at least anapproximation to NTF, and perhaps exactly equal to NTF. Accordingly, theoutput of the third functional block 460 represents the component ofD_(out) that is primarily due to the A/D converter feedback loopoperating on the digital dither input.

The output of the third functional block 460 is applied to the negativeinput of a difference circuit 470. D_(out) is applied to the positiveinput of the difference circuit 470. Accordingly, the output of thedifference circuit 470 is a signal indicative of D_(out) with the signalcomponent of D_(out) due to the digital dither signal at leastsubstantially removed.

FIG. 5 is a diagram showing a second order analog-to-digital convertercircuit 500 implementing noise cancellation, in accordance with variousaspects of the present invention. The circuit 500 comprises a secondorder sigma-delta analog-to-digital converter circuit configuration. Forexample and without limitation, the first portion 510 of the exemplarycircuit 500 may correspond to a specific example of one of many possiblecircuit configurations for the signal-processing module 310 illustratedin FIG. 3. Also, for example and without limitation, the second portion550 of the exemplary circuit 500 may correspond to a specific example ofone of many possible circuit configurations for the filter module 350illustrated in FIG. 3. Accordingly, the scope of various aspects of thepresent invention should by no means be limited to aspects of thespecific circuit 500 illustrated in FIG. 5.

The exemplary A/D converter circuit 500, for the purpose of illustratingvarious aspects of the present invention, is generally similar to theA/D converter circuit 400 illustrated in FIG. 4. Various differences maycomprise, for example, an NTF value in the third functional block 560 of(1−z⁻¹) or any other transfer function versus the illustrated NTF of(1−z⁻¹)². This should, however, by no means limit the scope of variousaspects of the present invention.

Additionally, the exemplary systems 300–500 illustrated in FIGS. 3–5generally comprise components described in terms of discrete timetransfer functions. This should by no means serve to limit the scope ofvarious aspects of the present invention to characteristics of discretetime systems. For example, various modules or components may becontinuous time or discrete time in nature.

FIG. 6 is a graph 600 illustrating an exemplary system performanceincrease from implementing noise cancellation in an analog-to-digitalconverter circuit, in accordance with various aspects of the presentinvention. The graph 600 generally plots SNDR (signal tonoise-and-distortion ratio) improvement on the vertical axis versussignal input amplitude on the horizontal axis. SNDR improvement wasdetermined as the difference between the response of ananalog-to-digital converter circuit utilizing a digital dither signal todisrupt tonal behavior, with and without utilizing various aspects ofthe present invention.

The graph 600 is presented herein as a specific example of generalimprovements that may result from utilizing various aspects of thepresent invention. Accordingly, the scope of various aspects of thepresent invention should not be limited by characteristics of the graph600 or by characteristics of exemplary systems analyzed to obtain thedata plotted in the graph 600.

The previous examples and illustrations present various exemplary systemcomponents and modules. Such components and modules may be implementedin a variety of ways, including but not limited to, hardware circuitry,a processor executing software instructions, analog circuitry, orvarious combinations thereof. For example, the various modules andcomponents may be incorporated into a single integrated circuit.Accordingly, the scope of various aspects of the present inventionshould by no means be limited by characteristics of particular hardwareor software implementations of the previously mentioned exemplarycomponents and modules.

In summary, aspects of the present invention provide a system and methodfor noise cancellation in a signal-processing circuit (e.g., ananalog-to-digital converter circuit). While the invention has beendescribed with reference to certain aspects and embodiments, it will beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted without departing from the scope ofthe invention. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. Therefore, it is intended that theinvention not be limited to the particular embodiment disclosed, butthat the invention will include all embodiments falling within the scopeof the appended claims.

1. An analog-to-digital converter comprising: a first module comprising:an analog input that receives an analog signal; a digital dither inputthat receives a digital dither signal; and a digital output that outputsa digital output signal comprising a first component that is primarily afunction of the analog signal, and a second component that is primarilya function of the digital dither signal; wherein the functionalrelationship between the digital dither input of the first module andthe digital output of the first module is characterized by a digitaldither transfer function; and a second module comprising: a digitalinput communicatively coupled to the digital output of the first module;a digital dither input that receives a signal indicative of the digitaldither signal; a first sub-module comprising a digital dither input thatreceives the signal indicative of the digital dither signal and adigital output, wherein the functional relationship between the digitaldither input of the first sub-module and the digital output of the firstsub-module is characterized by a transfer function that is based on thedigital dither transfer function of the first module; and a digitaloutput that outputs a digital output signal reflective of the digitaloutput signal from the first module with the second componentsubstantially removed.
 2. The analog-to-digital converter of claim 1,wherein the first module comprises a delta-sigma modulator.
 3. Theanalog-to-digital converter of claim 1, wherein the digital dither inputof the second module is communicatively coupled to the digital ditherinput of the first module.
 4. The analog-to-digital converter of claim1, wherein the first module comprises an A/D converter module, and thesecond module comprises a filter module.
 5. The analog-to-digitalconverter of claim 1, wherein the transfer function of the firstsub-module is substantially equal to the digital dither transferfunction of the first module.
 6. The analog-to-digital converter ofclaim 1, wherein the second module further comprises a second sub-modulecomprising: a first input communicatively coupled to the digital outputof the first module; a second input communicatively coupled to thedigital output of the first sub-module; and an output that outputs asignal based on the difference between the digital output of the firstmodule and the digital output of the first sub-module.
 7. A method forconverting an analog signal to a digital signal, the method comprising:inputting an analog signal to an A/D circuit; inputting a digital signalto the A/D circuit; outputting a digital signal from the A/D circuit,wherein the digital signal output from the A/D circuit is a function ofthe analog signal input to the A/D circuit and the digital signal inputto the A/D circuit; estimating a portion of the digital signal outputfrom the A/D circuit that is due to the A/D circuit processing thedigital signal input to the A/D circuit, wherein the estimatingcomprises determining a transfer function for the A/D circuit thatcharacterizes the functional relationship between the digital signalinput to the A/D circuit and the digital signal output from the A/Dcircuit, and applying the transfer function to the digital signal inputto the A/D circuit; and removing the estimated portion from the digitalsignal output from the A/D circuit.
 8. The method of claim 7, whereinthe A/D circuit comprises a delta-sigma modulator.
 9. The method ofclaim 7, wherein inputting a digital signal into the A/D circuitcomprises inputting a digital dither signal into the A/D circuit.
 10. Asignal-processing circuit comprising: a signal-processing modulecomprising a first input, a digital input and and output, wherein: thesignal-processing module outputs a signal that is a function of a signalinput at the first input and a digital signal input at the digitalinput; and the functional relationship between the digital input and theoutput is characterized by a digital transfer function; and a filtermodule communicatively coupled to the signal-processing module, thefilter module comprising: a first sub-module comprising a digital inputand an output, wherein: the digital input is communicatively coupled tothe digital input of the signal-processing module; and the functionalrelationship between the digital input and the output is characterizedby a digital transfer function that is based on the digital transferfunction of the signal-processing module; and a second sub-modulecomprising a first input, a second input and an output, wherein: thefirst input is communicatively coupled to the signal-processing moduleoutput; the second input is communicatively coupled to the firstsub-module output; and the second sub-module outputs a signal that is afunction of the difference between a signal received at the first inputand a signal received at the second input.
 11. The signal-processingcircuit of claim 10, further comprising a digital signal generatormodule that comprises a digital output communicatively coupled to thesignal-processing module digital input and the first sub-module digitalinput, wherein the digital signal generator module outputs a digitaldither signal.
 12. The signal-processing circuit of claim 10, furthercomprising a digital signal generator module that comprises a digitaloutput communicatively coupled to the signal-processing module digitalinput and the first sub-module digital input, wherein the digital signalgenerator module outputs a digital signal designed to reduce tonal noisein the signal-processing module.
 13. The signal-processing circuit ofclaim 10, further comprising a digital signal generator module thatcomprises a digital output communicatively coupled to thesignal-processing module digital input and the first sub-module digitalinput, wherein the digital signal generator module outputs a digitaldither signal.
 14. The signal-processing circuit of claim 10, whereinthe signal-processing module receives an analog signal at the firstinput of the signal-processing module and outputs a digital signal. 15.The signal-processing circuit of claim 10, wherein the signal-processingmodule comprises an analog-to-digital converter, the signal-processingmodule receives an analog input signal at the first input of thesignal-processing module, and the signal-processing module outputs asignal comprising a digital representation of the analog input signal.16. The signal-processing circuit of claim 10, wherein the digitaltransfer function of the first sub-module is substantially identical tothe digital transfer function of the signal-processing module.
 17. Thesignal-processing circuit of claim 10, wherein the digital transferfunction of the first sub-module is an approximation of the digitaltransfer function of the signal-processing module.
 18. Thesignal-processing circuit of claim 10, wherein the first modulecomprises a delta-sigma modulator circuit.
 19. A signal-processingcircuit comprising an analog-to-digital converter that generates adigital signal comprising a first component that is at least a functionof an analog input signal and a second component that is at least afunction of a digital input signal, said analog-to-digital converteroutputs a digital output signal that is representative of the generateddigital signal having the second component substantially removedwherein: the analog-to-digital converter comprises a filter module thatreceives the generated digital signal and substantially removes thesecond component from the generated digital signal: the filter modulereceives the digital input signal and estimates the second component ofthe generated digital signal based at least in part on the digital inputsignal; and the functional relationship between the digital input signaland the generated digital signal is characterized by a digital transferfunction, and the filter module estimates the second component of thegenerated digital signal based at least in part on an estimation of thedigital transfer function.
 20. The signal-processing circuit of claim19, wherein the second component is at least a function of a digitaldither signal.
 21. The signal-processing circuit of claim 19, whereinthe analog-to-digital converter comprises a delta-sigma modulatorcircuit.
 22. The signal-processing circuit of claim 19, wherein theestimation of the digital transfer function is substantially identicalto the digital transfer function.
 23. The signal-processing circuit ofclaim 19, wherein the estimation of the digital transfer function isidentical to the digital transfer function.
 24. A method for convertingan analog signal to a digital signal, the method comprising: inputtingan analog signal to an A/D circuit; inputting a digital signal to theA/D circuit; outputting a digital signal from the A/D circuit, whereinthe digital signal output from the A/D circuit is a function of theanalog signal input to the A/D circuit and the digital signal input tothe A/D circuit; estimating a portion of the digital signal output fromthe A/D circuit that is due to the A/D circuit processing the digitalsignal input to the A/D circuit, wherein the estimating comprisesestimating a digital transfer function for the A/D circuit thatapproximately characterizes the functional relationship between thedigital signal input to the A/D circuit and the digital signal outputfrom the A/D circuit, and applying the estimated digital transferfunction to the digital signal input to the A/D circuit; and removingthe estimated portion from the digital signal output from the A/Dcircuit.
 25. The method of claim 24, wherein the A/D circuit comprises adelta-sigma modulator.
 26. The method of claim 24, wherein inputting adigital signal into the A/D circuit comprises inputting a digital dithersignal into the A/D circuit.